Electrical transmission lines are used to transmit electric energy and signals from one point to another. The basic transmission line connects a source to a load--e.g. a transmitter to an antenna, an antenna to a receiver, or any other application that requires a signal to be passed from one point to another in a controlled manner. Electrical transmission lines, which can be described by their characteristic impedance and their electrical length, are an important electric component in radio frequency (RF) circuits. In particular, transmission lines can be used for impedance matching--i.e., matching the output impedance of one circuit to the input impedance of another circuit. Further, the electrical length of the transmission line, typically expressed as a function of signal wavelength, determines another important characteristic of the transmission line device.
Manipulation of the characteristic impedance and electrical length of the transmission line device is a well known technique to effect a particular electrical result. In particular, an output impedance, Z.sub.out, can be matched to an input impedance, Z.sub.in, according to a well known equation, as later described. Similarly, the attenuation and phase shift of the transmission line device can be altered by changing the physical length of the conductor between the input and output ports of the transmission line device. As an example, a resonant circuit results when the physical length of the conductor approximates an even one-quarter wavelength of the signal's nominal frequency.
Of course, at high frequencies the wavelength is small and transmission line devices can be built using relatively short conductors in small packages. By contrast, as the nominal frequency of the applied signal decreases, the physical length must necessarily increase to effect the desired transmission line characteristic. The physical length must correspondingly increase to accommodate such applications operating at lower frequencies.
Prior art techniques, including microstrip and stripline conductors, have been used successfully in the past to construct transmission line devices. Unfortunately, at lower frequencies--e.g., below 1 GHz13 the substrates upon which these one-dimensional conductive strips are placed require a relatively large area, due to the excessive length requirements. As today's electronic devices shrink in size, the board space allotted for the necessary electrical components is correspondingly reduced. Thus, a substrate carrying a microstrip or a stripline conductor that serves as a transmission line device for low frequency signals simply cannot be accommodated by the available board space.
Another technique that is employed can be described as a helical structure disposed inside a grounding cylinder. Such helical coils are well known in the art, but these too are often inadequate for today's applications, where low volume and low cost are critical factors in the manufacture of portable electronic devices. Because of the tight length and impedance specifications, the helical structures become very costly to manufacture. That is, the manufacturing variance that is inherent in the construction of such devices--e.g. conductor diameter, symmetry of windings, and effective number of turns--tends to make the helical structure a less desirable solution for tight tolerance transmission line devices. Further, the cylindrical grounding portion, which feature is required when building a transmission line device, results in a circuit having a relatively large volume, or poor form-factor, that is untenable for many of today's applications.
Of course, circuits that employ more than one transmission line device--or one having a relatively long electrical length requirement--suffer from increasing gains in volume associated with prior art devices. Such circuits include complex impedance transformers, quarter-wave resonators, and power splitters/combiners that require two or more transmission line devices to effect the desired result. Aside from the increased area and volume requirements, circuits employing multiple transmission lines on a single substrate must account for undesired interference between the two devices that might substantially effect their performance. In particular, if the transmission line structures are placed substantially adjacent to one another, an undesired coupling effect occurs that results, inter alia, in a reduced quality factor (Q), and reduced control of the characteristic impedance of the devices. Further, when the devices are positioned substantially parallel to each other, the result is an undesired coupling of the electromagnetic fields radiating from each of the structures. This undesired coupling often results in additional loss and performance degradation.
Accordingly, there exists a need for a circuit configuration capable of employing multiple, or lengthy, transmission line devices in a manner that reduces the volume requirements as compared to prior art circuits. Further, such a circuit that also oriented the transmission line devices in such a manner so as to substantially eliminate any undesired coupling effects between them would be an improvement over the prior art.